take look at my solution
i want to make visaCard-note
div to be visible only if selected cardType
is visa
and here is the html
<select name="cardType">
<option value="1">visa</option>
<option value="2">mastercard</option>
</select>
here is the js
var visa="1";//visa is selected by default
$("select[name=cardType]").change(function () {
document.getElementById('visaCard-note').style.visibility = this.value==visa ? 'visible' : 'hidden';
})